The invention has been developed primarily in connection with an agricultural implement of the fully mounted type, but it may also be applied to an implement of the semi-mounted type in which the forward part of the implement is supported by the lifting mechanism at the rear of the propelling vehicle, whereas further parts of the implement are supported by ground wheels. The main implement of this type to which the invention may be applied, with particular advantage, is a direct drilling implement (a seed drill), in which it is important to ensure, as far as possible, that the seed is introduced into the ground at a substantially constant depth below ground level, despite possible undulation in the surface profile of the field which is being drilled.
A direct drilling implement is capable of introducing seed directly into unprepared ground. It is not necessary to plough or work the soil before the drilling machine is used. Therefore, to achieve direct drilling on unprepared ground, the implement normally has disks mounted at its front end, and immediately behind them are grubber tines which cut down into the ground and loosen the top layer of the soil down to the depth where the seed is to be introduced. The machine also has rollers at the rear end of the machine which press and pack the loose soil over the seed, after introduction into the ground.
Depth control of the machine is usually carried out by use of depth wheels. The purpose of these wheels is to prevent the tools which loosen the soil from penetrating too deeply into the ground. However, when the soil is hard, the working tools of existing implements do not have sufficient force to penetrate deeply into the ground. The seed drills are usually mounted on a second frame, which is pivotally connected to the main frame. This second frame is pressed down onto the ground surface by a number of springs, and such springs have to be strong enough to hold the second frame in position. The rollers at the rear end of the implement prevent the second frame, with its seed drills, from penetrating too deeply. The seed drills are usually positioned on tines that are resiliently connected to the second frame.
Therefore, in existing machines, depth control is achieved by use of the main depth wheels, and the rear rollers hold the implement in a required position. However, it is a matter of practical experience that existing machines are not always capable of maintaining the seed drills at a required depth, owing to variable hardness of the ground surface and/or variable undulation in the ground surface.
Failure to maintain a substantially constant drilling depth will result in uneven germination of the seed, or possibly no germination at all at certain parts of the field.
Given that the ground level of most fields has a varying profile e.g. undulating in the form of successive humps and hollows (and together with varying ground hardness causing variable penetration by the grubber tines etc), it is difficult to maintain a substantially constant depth of drilling using existing implements, in that a tractor/implement combination will tend to oscillate (in a vertical plane) during transit across a field, and which tends to result in the height of the "seed drills" varying out of phase with this oscillation, resulting in variation in the height of the seed drill relative to the immediately underlying ground surface, with resulting variation in depth of introduction of seed into the ground.
At the very least, this can result in different rates at which the germinated seed pushes upwardly through the ground surface, and at the worst results in a total failure of the seed to germinate.